1. Field of the Invention
This invention relates to a harmonic friction drive apparatus.
2. Prior Art
Harmonic gear drive transmissions are typically used in industrial robots, machine tools, printing presses, medical equipment, communications equipment, solar energy applications, and high precision electronic component production. They can provide high ratios of gear reduction in a single stage compact arrangement.
There are three basic components used in a harmonic drive. A circular rigid ring, a flexible ring which is a non-rigid thin cylindrical cup of slightly smaller diameter than the rigid ring and a wave generator which is fitted within the cylindrical cup to elastically deflect it against the circular ring. The flexible ring is slightly smaller in diameter than the rigid ring. In a typical approach the rigid ring includes internal gear teeth which engage external gear teeth on the smaller diameter flexible ring. The flexible ring has fewer teeth on its outer circumferential surface than the rigid ring has on its inner circumferential surface. The conventional wave generator typically has an elliptical shape so that the teeth of the rigid ring and flexible ring engage across the major elliptical axis of the harmonic gear drive. In this approach each turn of the wave generator moves the rigid ring a few teeth backwards on the flexible ring. As the wave generator rotates the zone of tooth engagement travels with the major elliptical axis.
A typical harmonic gear drive using a flexible cup type flexible ring is described in U.S. Pat. No. 5,269,206 to Kiyosawa et al. Further examples of harmonic gear drives can be found in U.S. Pat. Nos. 6,119,553 to Yamagaishi et al., 6,082,222 to Kiyosawa, 6,065,362 to Kiyosawa et al., 5,906,142 to Shirasawa, 5,850,765 to Shirasawa and 5,775,178 to Asawa et al. Each of these patents describes harmonic drives and various features of such drives. These patents are specifically incorporated by reference herein.
Further information concerning typical harmonic gear drives can be found in a catalog number 9705-2R-MINI put out by HD Systems, Inc., 89 Cabbot Court, Hauppauge, N.Y. 11788.
The harmonic gear drives described above employ a gear reduction approach wherein gear teeth are placed on both the external surface of the flexible ring and the internal surface of the rigid ring. Because of the use of gear teeth problems arise in miniaturizing such devices, since it is difficult to make the teeth of the gearing small enough to allow shrinking the size of the harmonic gear drive.
There is a need for an improved harmonic drive, which will provide high ratios of speed reduction or speed increase in a single stage. There is a need to significantly increase the upper limit of the reduction ratio or speed increase ratio which can be obtained, all at a reasonable cost. It is also desired to provide such a harmonic drive which due to its simplicity may be scalable to the MEMS (micro electromechanical systems) level.
In accordance with this invention there is provided an improved harmonic friction drive which can provide high ratio, speed reduction, speed increase, functions or function as a differential or provide various combinations of such functions. The device operates similarly to a conventional harmonic gear drive for carrying out those functions, except that it utilizes friction instead of gear teeth to transfer energy from a flexible member to a rigid outer ring, using a wave generator.
In accordance with a preferred embodiment of this invention a harmonic friction drive is provided comprising the following components. A rigid ring having a cylindrical inside surface of a given circumference, the inside surface being free of gear teeth. A member having a flexible tube portion, the flexible tube portion being nested in the rigid ring. The flexible tube portion has an outer surface whose circumference is smaller than the given circumference. The outer surface of the flexible tube portion is also free of gear teeth. A wave generator is arranged to push the outer surface of the flexible tube portion into frictional engagement against the inside surface of the rigid ring, at two or more zones along the inside surface. The wave generator rotates the two or more zones of engagement about the inside surface of the ring. The difference in circumference between the outside surface of the flexible tube portion and the inside surface of the ring causes the flexible tube portion to rotate a small amount relative to the rigid ring upon each complete revolution of the wave generator.
This invention is aimed at reducing cost by eliminating the gear teeth of a conventional harmonic gear drive thereby eliminating costly machining operations. It is also aimed at eliminating the barrier associated with the use of gearing with respect to making small scale versions of this drive.